This invention relates generally to ultrasonic systems for measuring the flow of liquid through a pipe, and more particularly to a pulse power generating arrangement for the upstream and downstream transducers of the system.
The use of ultrasonic techniques to determine the flow rate of a liquid flowing through a pipe is well known. In a system of this type, such as those disclosed in U.S. Pat. Nos. 4,103,551; 4,004,461 and 3,906,791, a pulse of acoustic energy is alternately emitted by a pair of ultrasonic transducers; one being placed upstream on the pipe at one end of a tilted diameter or diagonal; the other being placed downstream at the other end of the diagonal, the direction of tilt lying in the direction of flow.
In operation, first a pulse of ultrasonic energy emitted by the upstream transmitter and propagated through the fluid in the pipe along a path at an angle to the longitudinal flow axis is detected by the downstream transducer. Then a pulse emitted by the downstream transducer propagated through the fluid along the same path is detected by the upstream transducer. The time delay difference (that is, the upstream time minus te downstream time) between the generation of the emitted pulses and their reception is a function of flow velocity and therefore is convertible into a flow rate reading.
In some ultrasonic flowmeter systems, the upstream and downstream transducers are clamped onto the pipe on the same side thereof, in which case the pulse emitted by one transducer is propagated through the fluid toward the opposite side of the pipe from which it is reflected toward the other transducer. But whether the two transducers are on the same side or on opposite sides of the pipe, the operation of the flowmeter depends on the time delay difference between the up-down and down-up transit times.
It has heretofore been the practice, in driving the upstream and downstream transducers of an ultrasonic flowmeter, to provide a separate high-voltage pulse generator for each transducer. Thus in the U.S. Pat. No. 4,195,517 of Kalinoski et al., whose entire disclosure is incorporated herein by reference, the upstream transducer is provided with a pulse-generating charge/fire circuit, and the downstream transducer is provided with a separate pulse-generating circuit. These circuits are triggered by means of pulses applied thereto at appropriately timed intervals by a digital microprocessor which also acts to measure the acoustic propagation times. Similarly, in the British Pat. No. 1,508,636, the upstream and downstream transducers are driven by separate power pulse generators to which start pulses are applied; the start pulses applied to one generator being displaced in time relative to those applied to the other to avoid a conflict between the pulses emitted by each transducer and the pulses received thereby.
We have found that when one compares the absolute times of the leading edges of the power pulses applied by separate generators to upstream and downstream transducers with reference to the start time of the system, that the upstream and downstream times are not exactly equal but are slightly displaced. This is due to differences in the turn-on time of the generators; for no two pulse power generators, though seemingly identical in circuit design, possess precisely the same turn-on time.
Thus it may be that the upstream pulse generator takes slightly more time to turn on than the downstream generator, this time displacement giving rise to a time difference between received ultrasonic signals. The system interprets the turn-on time displacement between the upstream and downstream pulse generators as a zero flow offset, this being added to the time difference factors of the acoustical coupling devices, the pipe, and fluid conditions in the pipe.
One can calibrate out this zero offset, but in doing so, one does not cancel the effect of temperature on the reading; for the upstream minus the downstream time difference at zero flow renders the system sensitive to temperature changes. Hence even a slight difference in the turn-on times of separate pulse power generators is detrimental to the accuracy of the system.
It is also to be noted that in many known types of ultrasonic flowmeters, the receiver is common to the upstream and downstream transducers, and that the microprocessor which includes logic to separate upstream and downstream pulses, to selectively gate the low-level transmit pulses and to perform measuring functions, is shared by the transducers, the only non-shared electronics in the system being the separate pulse power generators for the transducers.